Current hydraulically-actuated fuel injectors typically include three main portions: a control portion, a hydraulic pressurizing portion, and a nozzle portion. The control portion typically includes a solenoid with an armature and one or more operably connected valve members. The hydraulic pressurizing portion typically includes an intensifier piston and plunger assembly movably mounted in a piston/plunger barrel. The nozzle assembly portion typically includes a spring biased needle valve member that opens and closes a nozzle outlet. Of these three portions, the control portion is typically the one that causes most technical problems, such as injector to injector variations, injector stability, seat cavitation power growth or loss, and noise. In order to resolve these problems, many special manufacturing techniques, such as coating, special heat treatment and other special machining processes have significantly increased the cost of hydraulically-actuated fuel injectors.
From a performance point of view, many hydraulically-actuated fuel injectors can not do a split injection using wave form control because the control valve cannot respond fast enough. In order to produce a split injection, some hydraulically-actuated fuel injectors spill an amount of fuel at the beginning of the injection event. However, this split injection through fuel spilling increases plunger stroke, which can cause some structural problems and can only be accomplished with an undesirable energy loss. In addition, the control valve poppet member lower seat flow restriction limits the pressure capability, and injection duration cannot typically be reduced by simply increasing actuation fluid rail pressure. Since the control valve's spring cavity works in an alternating mode from high pressure to low pressure, lower seat cavitation is sometimes observed in hydraulically-actuated fuel injectors operating at idle condition with a high rail pressure. Because the injector has to be charged with high pressure actuation fluid during each injection event, yet be released from the high pressure between each injection, the timing for the charge and release is controlled by the movement of a poppet control valve member. It has been observed that the valve member moves slower at high rail pressure, causing the injection rate to ramp up more slowly and decay slowly. Consequently, it is often difficult for many hydraulically-actuated fuel injectors to produce a square injection rate profile. This same slowing of the poppet control valve member is often the reason why it is very difficult to reduce injection duration for relatively small high speed fuel injectors because the injection event mainly occurs during the brief poppet motion from its lower seat, to the upper seat, and back to its lower seat. This poppet control valve member slowing can also be the source of a reduction in mean effective injection pressures for high speed fuel injectors, even when peak injection pressure is relatively high.
In an effort to address some of these problems, some hydraulically-actuated fuel injectors have incorporated direct control needle valves in their operation. A direct control needle valve includes a needle valve member with a closing hydraulic surface, which can be exposed to either low or high pressure. The direct control needle valve allows the nozzle outlet to be held closed while fuel pressure builds within the injector, permits some split injection capabilities and rate shaping. In addition, these injectors often have the ability to abruptly close the nozzle outlet, even in the presence of highly pressurized fuel at injection pressures. In order for these hydraulically-actuated direct control needle fuel injectors to be a viable alternative to their predecessors, they typically must have the ability to accomplish their additional tasks without including an additional electronic actuator. While the inclusion of a direct control needle valve has proven realistic, new complications must necessarily develop due to the inclusion of additional high speed moving parts within the injector and the highly dynamic nature of component movements and fluid pressures within the injector during each injection event. In any event, many of the performance concerns associated with charging and releasing high pressure on the top of the intensifier piston within a hydraulically-actuated fuel injector remain with or without the incorporation of a direct control needle valve.
The present invention is directed to overcoming these and other problems associated with hydraulically-actuated fuel injectors that charge and release high pressure on the top of an intensifier piston during each injection cycle.